The presence of undesired signals is a concern in the processing of virtually all electromagnetic signals. Even in a relatively simple system, such as a radio receiver, a squelch control is often provided to attenuate signals below a certain magnitude, so as to avoid undesired background static being audible when a signal of interest is not being received. What constitutes undesired background static is for the user to judge, and the user can set the squelch control level to limit the audibility of received signals based on the user's judgment.
Automated signal processing systems, where a computer system autonomously responds to input signals, present a more difficult problem. Unlike the example of a squelch control noted above, where a user can adjust the squelch control level based on experience and judgment, it is more difficult to program a computer system to automatically set a limit to differentiate between certain types of signals that are desirable and those that are not. For example, computers respond well to unambiguous input from keyboards, pointing devices, and similar input devices, but respond less satisfactorily to voice commands. Anyone who has used speech recognition programs has experienced some difficulty when the computer fails to recognize something the user has said. As might be expected, the computer's failure to accurately recognize a user's speech happens more frequently in the presence of background noise or other sounds that affect the overall auditory input perceived by the computer.
Computer vision recognition of objects represents another difficult problem. If the computer must process too much visual data or too broad a range of visual signals, the input will more likely be misread and incorrectly interpreted by the computer. On the other hand, if the computer improperly suppresses visual signals that are needed to properly perceive objects, the computer also may misread visual inputs or ignore important visual input entirely. Computer vision is becoming increasingly more important in making computers and their interfaces even more user friendly. For example, the MIT Media Lab, as reported by Brygg Ullmer and Hiroshi Ishii in “The metaDESK: Models and Prototypes for Tangible User Interfaces, “Proceedings of UIST 10/1997:14-17,” has developed a form of “keyboardless” human-machine interface that employs computer vision. The metaDESK includes a generally planar graphical surface that not only displays computing system text and graphic output, but also receives user input by “seeing” and responding to an object placed on the graphical surface. The combined object responsive and display capability of the graphical surface of the metaDESK is facilitated using infrared (IR) lamps, a video camera or other image sensor, a video projector, and mirrors disposed beneath the surface of the metaDESK. The mirrors reflect the graphical image projected by the projector onto the underside of the graphical display surface to provide images that are visible to a user from above the graphical display surface. The IR camera can detect IR reflections from the undersurface of an object placed on the graphical surface. By “seeing” and detecting a specially formed object or IR-reflected light from an object disposed on a graphical display surface, the metaDESK can respond to the contemporaneous placement and movement of the object on the display surface, to carryout a predefined function, such as displaying and moving a map of the MIT campus that is displayed on the surface of the metaDESK.
Others have been developing similar keyboardless interfaces. For example, papers published by Jun Rekimoto of the Sony Computer Science Laboratory, Inc., and associates describe a “HoloWall” and a “HoloTable” that display images on a surface and use IR light to detect objects positioned adjacent to the surface.
Both the metaDESK and HoloWall/HoloTable use IR light to see objects and detect the movement of the objects for several reasons. If the systems responded to visible light, the visible light projected by the systems to produce images, which would be partially reflected back by the interactive surface could lead to false readings by the computer vision system. Further, even if such reflections could be suppressed, unless the system were disposed in a dark room, room light and other visible light passing through the interactive display surface would adversely affect the computer vision systems. Furthermore, if such a system were configured to respond to visible light, the system could not produce dark or dim screens because there either would not be sufficient visible light to detect objects and movements, or the light used to detect objects and movement would eclipse the dark or dim images intended for the user to see.
Using IR light or other non-visible light outside the visible spectrum, such as ultraviolet (UV) light, to detect objects placed on an interactive display surface can avoid some of the problems that would arise from attempting to recognize objects with visible light. However, because various visible light sources also produce UV and/or IR light, light from such ambient sources can also adversely impact computer vision systems. For example, incandescent lights, the sun, and a variety of other common sources generate IR and/or UV light. These unintended IR signals, just like unintended visible light signals, can provide undesired input to non-visible-light-sensitive computer vision systems. Band-pass type filters can suppress undesired bandwidths of light generally, but they are not helpful in separating light from a controlled source that is reflected from an object, from ambient light that happens to include non-visible light.
One way to suppress the effects of uncontrolled light sources is to selectively control illumination from a controlled light source, and capture frames of image data with the controlled light source alternately turned on and off. Frames captured with the controlled light source turned off represent data resulting from any uncontrolled light sources that produce non-visible light. Subtracting the light intensity detected in the frames captured with the controlled light source off can thus be used to compensate for the effects of uncontrolled light sources. Such a process is explained in a co-pending, commonly assigned U.S. patent application entitled “method And System For Reducing Effects Of Undesired Signals In An Infrared Imaging System,” Ser. No. 10/870,777, filed on Jun. 16, 2004. Such a system may be workable in many contexts.
However, in situations where the imaging system is imaging moving bodies such a time-slicing system may be less optimal. Using such a time-slicing system reduces the effective number of frame capture cycles actually available for data capture. Thus, for example, if a system is configured to capture frames with the controlled light source alternately turned on and then off, the effective capture rate of the imaging system for vision data is only one-half that of a system that is always capturing vision data. For example, alternating the capture of frames with the controlled light source turned on and then off, the effective capture rate of a digital camera capable of capturing 30 frames per second is effectively reduced to 15 frames per second. Moreover, by capturing image data less frequently, the system becomes more susceptible to read errors that may arise when the object being imaged moves during intervals when the controlled light source is turned off. Thus, in an image capture system sampling image data at a reduced rate in order to use a portion of available capture cycles to compensate for ambient light, resulting image data may be choppy, distorted, and/or miss significant object movements.
It is therefore desirable to filter, mask, or otherwise reduce the effects of unintended and undesired light signals, to prevent a vision system from responding to extraneous light signals having light in the same waveband as used by the vision system. The effect of the undesirable background light should be avoided when detecting objects without requiring that a computer vision system be operated in an environment that shields it from all background light sources. Moreover, it is desirable to reduce the effect of light produced by unintended and undesired light sources without sacrificing image capture cycles, which can reduce the ability to accurately capture data resulting from objects moving in the field of interest.